1. Field of the Invention
The present invention relates to a process for preparing oxide thin films having excellent flatness and high crystallinity, and more specifically to an improved MBE (Molecular Beam Epitaxy) process so called a reactive co-evaporation process particularly for preparing thin films of oxide insulator or dielectric mateials, which have clean and smooth surfaces, high crystallinity and excellent properties.
2. Description of Related Art
Oxide thin films have been widely used as insulating layers and protective layers for semiconductor devices. Oxide thin films are also used as insulating layers and barrier layers of superconducting devices utilizing oxide superconductor materials which have been recently advanced in study, for example, Y-Ba-Cu-O type oxide superconductor having a critical temperature higher than 80K, Bi-Sr-Ca-Cu-O type oxide superconductor and TI-Ba-Ca-Cu-O type oxide superconductor having critical temperatures higher than 100K.
Since interactions among superconducting particles, such as Cooper pairs, through insulating layers are necessary for operating the superconducting devices and to high breakdown voltage is required for the insulating layers of control electrodes, the oxide thin films utilized in the superconducting devices should be more homogeneous and should have higher crystallinity and smoother surfaces than that of a semiconductor device.
In addition, the superconducting devices often include stacked structures formed of oxide superconductor thin films and nonsuperconductor thin films. For example, a tunnel type Josephson junction comprises a stacked structure of a first oxide superconductor layer, an thin insulator layer and a second oxide superconductor layer stacked in the named order.
A superconducting field effect device, a candidate of realistic three-terminal superconducting devices, has a superconducting channel and a gate electrode formed on the superconducting channel through a gate insulating layer. If a superconducting field effect device consists of a superconducting channel of an oxide superconductor, it is necessary to stack an oxide superconductor thin film, an insulator thin film and a conductor film.
In order to fabricate a superconducting multi-layer wiring structure by using an oxide superconductor, oxide superconductor thin films and insulator thin films should be alternately stacked.
All of above superconducting devices and elements require high quality thin films. Namely, oxide superconductor thin films and other thin films preferably should have excellent properties originated by high crystallinity. It is more preferable that the thin films are formed of single crystals. If one of the thin films is formed of polycrystals or amorphous state, the device or the element may have degraded characteristics or may not even operate.
Additionally, in the above superconducting devices and elements, junctions between the two thin films are also important. It is preferable that there is no inter diffusion through the junction interfaces, therefore, there should exist clear interfaces. In particular, if constituent elements of the nonsuperconductor thin films diffuse into the oxide superconductor thin films, superconducting properties of the oxide superconductor thin films are fairly degraded, and vice versa.